This invention relates generally to torque sensors, and in particular to non-compliant torque sensors utilizing a magnetoelastic element and a non-contacting magnetometer for sensing magnetic field changes that correspond to changing torque values in a rotatable shaft.
Sensing the torque of rotating shafts is desirable in many applications, such as determining steering wheel effort measurements in electronic power steering systems, determining transmission output torque for electronically controlled shifting, determining power tool output torque, and the like. Torque sensors have been produced in many varieties, and can be generally classified as xe2x80x9ccompliantxe2x80x9d and xe2x80x9cnon-compliantxe2x80x9d torque sensor types. In so-called compliant torque sensors, a sensor, such as a strain gauge, is attached directly to an elastic beam section of a torque-producing shaft. When torque is applied to the elastic beam section, the strain gauge is deflected, which causes a resistance change in the strain gauge. This change of resistance in the strain gauge indicates a change in torque. However, due to the rotating nature of the beam section to which the strain gauge is attached, connecting wires to the strain gauge for transmission of signals is impractical. Thus, the strain gauge type torque sensors require a wireless transmitting device, such as a radio-frequency transmitter, to transmit resistance changes in the strain gauge to a receiver, which interprets these signals as torque values. Alternatively, a signal transference scheme utilizing slip rings, brushes and commutators could be used in a compliant torque sensing system.
However, such compliant torque sensing systems present numerous problems. For instance, because the strain gauges are attached directly to an elastic beam, torque limiters must be included on the rotating shaft to protect the beam and strain gauges from being deflected beyond their elastic range. Unfortunately, such precautions inherently interfere with the transmission of energy through the shaft, and, in the instance of a steering wheel shaft, provide a xe2x80x9csoft feelxe2x80x9d to the user. Additionally, such torque sensors are of limited reliability due to the direct contact with the rotating shaft, and are very expensive. The strain gauge type torque sensors also require frequent calibration.
To overcome these problems, non-compliant torque sensors were developed, whereby a sensor monitors shaft torque changes in a non-contacting manner, thus obviating the need for torque limiters. Normally, such torque sensors utilize a magnetoelastic element intimately attached to a rotating shaft, whereby the torque sensor would operate on the principle of inverse-magnetostriction.
Magnetostriction is well known and describes a structural property of matter that defines a material""s dimensional changes as a result of a changing magnetic field. In essence, magnetostriction is caused when the atoms that constitute a material reorient in order to align their magnetic moments with an external magnetic field. This effect is quantified for a specific material by its saturation magnetostriction constant, which is a value that describes a material""s maximum change per unit length.
Contrariwise, inverse-magnetostriction defines changes in a material""s magnetic properties in response to applied mechanical forces Torque sensors that utilize inverse-magnetostriction operate on the premise that stresses and strains that are transmitted through the rotating shaft to the magnetoelastic element by the application of torque cause measurable changes in the magnetic field of the magnetoelastic element. Thus, the magnetic field strength produced from the magnetoelastic element is a direct function of the magnitude of the torque applied. A torque sensor utilizing such a magnetoelastic element would also have a magnetometer that would translate the magnetic field strength emanating from the magnetoelastic element into an analog voltage signal, thereby performing a torque to voltage transducer function. It is known in non-compliant torque sensors to attach a ring of magnetoelastic material to a rotating shaft via interference fitting means, such as a pressure fit or shrink fit, inter-engaging means such as mating splines or teeth, chemical means such as the use of an adhesive, thermal means such as thermal spraying, or any other type of attaching means as are known in the art. In practice, under any of the above attaching methods, the attachment of the magnetoelastic element to the shaft has proven to be of the utmost importance. Indeed, defects in the boundary between the magnetoelastic element and the torque carrying member will result in aberrant coupling of stress and strains into the magnetic element, which adversely affect torque measurements. Boundary defects can include imperfections such as voids, contaminates, and lateral shearing.
Further, practical requirements for torque sensors include design tolerance limits on the accuracy and linearity of the in-range voltage output and the amount of hysteresis, also known as xe2x80x9czero shift,xe2x80x9d after a xe2x80x9cyield torquexe2x80x9d or xe2x80x9cover-torquexe2x80x9d is applied to the shaft. Such xe2x80x9cover-torquexe2x80x9d conditions can exist, for example, in steering systems during curb push-away situations, and can be experienced in transmission applications during drastic torque reversals. Hysteresis may occur because after the over torque condition is relaxed, the resulting breakdown or slippage at the shaft/magnetoelastic element interface causes a mechanical bias in the magnetoelastic element. Consequently, a corresponding magnetic bias is produced, thereby negatively affecting future torque measurements. Further, if the breakdown of the shaft/magnetoelastic element interface is localized, the result may be a magnetic incongruity that manifests as a variance in torque measurements with respect to the angular position of the shaft. While such breakdown between the shaft and magnetoelastic element is normally not a problem where the magnetoelastic element is thermally sprayed, hysteresis still occurs in thermal sprayed magnetoelastic elements due to the different coefficients of thermal expansion between the shaft and magnetoelastic element, as will be explained in more detail herein.
For example, in an automotive steering column torque sensor, it is preferred that there be a full range torque measurement of +/xe2x88x92 6 ft-lb, and a hysteresis requirement +/xe2x88x92 1.5% of full scale after application of a 100 ft-lb yield torque. However, present thermal sprayed magnetic elements will exhibit hysteresis well over the acceptable limits even when a yield torque of only 15 ft-lb. is applied.
Thus, there is a need for a torque sensor that will exhibit low hysteresis after a yield torque is applied. Further, there is a need for a method of producing such a low hysteresis torque sensor.
Other needs will become apparent upon a further reading of the following detailed description taken in conjunction with the drawings.
In one form of the invention, the aforementioned needs are fulfilled by a low hysteresis torque sensor and method for producing a low-hysteresis magnetoelastic element comprising thermally spraying a magnetoelastic material onto a metal substrate. During the spraying process, compressive axial pressure is applied to the substrate, and subsequently released after the substrate and magnetoelastic element have substantially cooled. This process has the effect of substantially reducing hysteresis and the axial stress in the magnetoelastic element that would normally occur due to the different coefficients of thermal expansion between the magnetoelastic element and the substrate. In a preferred embodiment, the magnetoelastic element comprises nickel and the substrate comprises stainless steel. It is further preferred that the substrate comprise a shaft and the magnetoelastic element comprise a circumferential ring intimately attached thereto.